Use the IP you assigned to the linux server. The router has it's own IP so don't use the router's IP. If your linux webserver is 192.1.168.102, you can use the same IP for the other domains you create. One reason to dedicate an IP to a spacific domain is if you require SSL for that domain.

These are two email storage formats. Maildir means, that every mail is stored in a separate file inside the maildir directory. Mbox means, that all emails are stored in one large file, the format of this file is named mbox.

How to decide if you need Mbox / Maildir:

ISPConfig supports both formats, but most POP3 and IMAP Daemons support only MBox or Maildir, so you have to chose the mail storage format in ISPConfig that is supported by your POP3 and IMAP Daemon.

For example, courier is one of the few Daemons that support Mbox and Maildir, in this case i recommend to use Maildir because it is more robust and scales better than mbox format.

The default format in Fedora and SuSe is Mbox (please correct me if i'am wrong).

Using the private IP has solved the problem of not being able to view second and subsequent sites

My next problem is that I am supposed to have authorative DNS servers for two sites and the DNS servers are supposed to be on a Public IP. However, if I enter the Private IP as above, the DNS server is not open to the Public.